Seeing as how you have not told us what your overall computing needs are it's not possible to make a buying suggestion. What will you be using your laptop for? Will you be mobile a lot? Do you have a budget? Etc.
Unless he expects to use it with an external display (when actually DOING design work).
Because otherwise, 13" would make no sense at all. Even 15" is tiny.
LAmike: if you're looking for a portable workstation that you don't plan to use on the road, possibly you might want to consider a Mac Mini instead? Should be a lot more bang for your buck, and it will run cooler and more stable for long-term intensive use. Downside is, you can't use it on the road.
But between two MBP...
the 15" will be a lot more usable as-is.
The 13" will probably last longer before its battery fails.
AppleCare might still be available for the 13"? If you're expecting to do serious work on the machine,
this could be a Good Thing, indeed.